Single compressor refrigeration systems are well known for cooling, freezing, storing, and transporting frozen and cooled products such as food, chemicals and other sensitive things like blood, human organs, etc. Generally, single compressor systems are inadequate for cooling a load to about -20.degree. F. (for example ice cream) at a high ambient temperature. Commercially available refrigeration systems having a single compressor or multiple compressors in parallel can cool a load to about 0.degree. F. Unfortunately, purchasers of refrigeration systems desire a system which can maintain a load at very low temperatures (e.g. -20.degree. F. and lower) at high ambient temperatures (e.g. 120.degree. F. and higher).
By way of example, consider a single compressor system for cooling a load to -20.degree. F. in an ambient environment of 120.degree. F. In this case, the necessary evaporator temperature is typically at least 10.degree. F. colder than the load temperature or -30.degree. F. Under these conditions using refrigerant R12, the evaporator pressure is expected to be approximately 12 psia and using R22, the expected pressure is approximately 20 psia. Similarly, the condenser temperature necessary to discharge heat to the ambient is about 10.degree.-40.degree. F. warmer than the ambient under the best case conditions (e.g. 130.degree. F.). Therefore, the pressure in the condenser should be approximately 196 psia for R12 and 312 psia for R22. This dictates a compression ratio of 196/12.apprxeq.16.5 for R12 and 312/20.apprxeq.15.6 for R22. Refrigeration compressors, however, are designed and built to operate with a compression ratio no greater than 10 to 15. If the pressure ratio exceeds the manufacturer's design criteria, the compressor will break. Accordingly, neither example above could be achieved with a conventional single compressor system. Indeed, a commercially available compressor can not operate under the above conditions and accordingly, such a system would be prohibitively expensive and inefficient. Thus, commercially available single compressor systems are incapable of operating where the difference between the desired product temperature and the actual ambient temperature is very large.
Compound compressor systems are well known. These systems typically comprise low and high stage compressors coupled together in series so that refrigerant flows through both of these compressors. It is well understood that in compound compressor systems the compression ratio is split between the low and high stage compressors, thereby allowing the system to achieve low evaporator pressure (i.e. low temperature) at high ambient temperatures. The compression ratio for the compound system is the product of the compression ratio for both the low and the high stage compressors. A compound system for the R22 example described above would also have a compression ratio of at least 16 and both the low and high stage compressors would operate at equal pressure ratios, i.e., approximately 4 for each compressor. This compression ratio is well within an acceptable range of the specifications of commercially available compressors and at these conditions the compressor efficiencies are quite high.
Cooling systems also require a minimum compression ratio to operate efficiently. As the difference between the product temperature and the ambient temperature is reduced, the compression ratio for the cooling system is also reduced. If the compression ratio becomes too low, the compressor capacity becomes too large and the compressor will short cycle and eventually break. In addition, the compressors in a compound system run less efficiently than a single stage compressor system when the difference between the load and ambient temperature decreases.
What is needed, therefore, is a system and method for cooling a load of product to a desired temperature which can efficiently operate over a broad range of ambient (-60.degree. F. to +120.degree. F.) and load temperatures (-25.degree. F. to +75.degree. F.).